This invention relates to electrical machines and, in particular, to cryogenically-cooled machines.
The invention is employed in electrical machines, e.g. motors, generators, converters, which are used at nuclear, steam and other power stations, as well as in vehicle transport and in aviation. The invention is advantageous to use also in other installations, where the shaft is to emerge from a cool zone into the environment.
At present much consideration is given to development of electrical machines, wherein the superconductivity phenomenon is realized, when the resistance in electric circuits approaches to its zero value. There exist materials capable of realizing this phenomenon at a temperature close to the absolute zero. That is why a cryogenic liquid, e.g. liquid helium, is used in such machines to cool the electric windings to such a temperature where they become superconductive (usually this temperature is about 5.degree. K).
Cryogenically-cooled electrical machines using a normal circuit comprise a superconductive winding secured in a rotor encased in a tight heat-insulating shell, with a protective vacuum being provided therein. The superconductive exciting winding has a cryogenic cooling system. The low-temperature zone of the superconductive exciting winding has to be heat-insulated from the ambient medium. Complete insulation of the machine drive system is impossible, because at least the drive end of the rotor shaft is to be extended outside. Usually, the shaft is solid, since it has to accept and to impart the torque. The shaft in this case brings in the main component of heat influx to the superconductive winding (up to 50 %). In this connection, there exists a problem of cooling the rotor shaft in order to build up such a thermal resistance between the winding and the rotor shaft as to limit the amount of heat propagating along the rotor axis to the winding.
There is known a cryogenically-cooled electrical machine in which in order to reduce the heat influx along the shaft to the superconductive winding of the rotor, a plurality of openings designed to remove the cooling agent from the rotor hollow space is made in places, where the central part of the rotor is attached to the shaft. The removed flow of the cooling agent works at the same time to cool the ends of the rotor shaft.
This design is deficient in that the efficiency of such cooling is low, since the heat removal area is small and cannot be expanded by mere increase of the number of openings due to the rotor shaft strength requirements.
There is also known a cryogenically-cooled electrical machine comprising a hollow rotor with a superconductive exciting winding, the supply bus conductors being coupled thereto. The cooling system comprising a passage for delivery of the cooling agent to the superconductive exciting winding, arranged along the axis of the rotor shaft, and a means for cooling the rotor shaft ends and passages for removing the cooling agent from the superconductive winding.
The means for cooling the rotor shaft ends comprise heat-insulating plugs installed in the central part of the hollow rotor, where it is joined to the rotor shaft. The fitted surfaces of the heat-insulating plugs and of the central part of the rotor are provided with helically arranged ribs.
The low-temperature gas or liquid (cooling agent) is supplied to the superconductive exciting winding through the passage made along the axis of the rotor shaft, from one end of said shaft.
The heat coming to the rotor hollow vaporizes the cooling agent, and the resulting vapors pass into the clearances between the internal surface of the rotor central part and the external surface of the heat-insulating plugs. The cold gas passes along the clearances between the ribs towards the cooling agent removal passages and takes away the heat penetrating from outside from the shaft butt ends.
In this machine, cooling of the rotor shaft ends is, however, ineffective, since a considerable amount of heat is transmitted to the rotor hollow in places, where the shaft ends are secured to the central part of the rotor and where no protection against the outside heat influx can be provided.
The additional means for cooling the ends of the rotor shaft, that is the heat-insulating plugs, makes the electrical machine design more complicated and larger.
Besides, such a design is deficient in that the gas which is heated when passing through the clearances between the ribs and through the passages for removal of the cooling agent is thrown inside the electrical machine, since it has no means for collecting the used cooling agent.
It should be also pointed out that the machine has no protection against heat influx along the supply bus conductors.